From: Michal Hocko <mhocko@suse.com>
To: Eiichi Tsukata <eiichi.tsukata@nutanix.com>
Cc: corbet@lwn.net, mike.kravetz@oracle.com, mcgrof@kernel.org,
keescook@chromium.org, yzaikin@google.com,
akpm@linux-foundation.org, linux-doc@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-fsdevel@vger.kernel.org, felipe.franciosi@nutanix.com
Subject: Re: [RFC PATCH] mm, oom: introduce vm.sacrifice_hugepage_on_oom
Date: Tue, 16 Feb 2021 09:12:33 +0100 [thread overview]
Message-ID: <YCt+cVvWPbWvt2rG@dhcp22.suse.cz> (raw)
In-Reply-To: <20210216030713.79101-1-eiichi.tsukata@nutanix.com>
On Tue 16-02-21 03:07:13, Eiichi Tsukata wrote:
> Hugepages can be preallocated to avoid unpredictable allocation latency.
> If we run into 4k page shortage, the kernel can trigger OOM even though
> there were free hugepages. When OOM is triggered by user address page
> fault handler, we can use oom notifier to free hugepages in user space
> but if it's triggered by memory allocation for kernel, there is no way
> to synchronously handle it in user space.
Can you expand some more on what kind of problem do you see?
Hugetlb pages are, by definition, a preallocated, unreclaimable and
admin controlled pool of pages. Under those conditions it is expected
and required that the sizing would be done very carefully. Why is that a
problem in your particular setup/scenario?
If the sizing is really done properly and then a random process can
trigger OOM then this can lead to malfunctioning of those workloads
which do depend on hugetlb pool, right? So isn't this a kinda DoS
scenario?
> This patch introduces a new sysctl vm.sacrifice_hugepage_on_oom. If
> enabled, it first tries to free a hugepage if available before invoking
> the oom-killer. The default value is disabled not to change the current
> behavior.
Why is this interface not hugepage size aware? It is quite different to
release a GB huge page or 2MB one. Or is it expected to release the
smallest one? To the implementation...
[...]
> +static int sacrifice_hugepage(void)
> +{
> + int ret;
> +
> + spin_lock(&hugetlb_lock);
> + ret = free_pool_huge_page(&default_hstate, &node_states[N_MEMORY], 0);
... no it is going to release the default huge page. This will be 2MB in
most cases but this is not given.
Unless I am mistaken this will free up also reserved hugetlb pages. This
would mean that a page fault would SIGBUS which is very likely not
something we want to do right? You also want to use oom nodemask rather
than a full one.
Overall, I am not really happy about this feature even when above is
fixed, but let's hear more the actual problem first.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-02-16 8:13 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-16 3:07 [RFC PATCH] mm, oom: introduce vm.sacrifice_hugepage_on_oom Eiichi Tsukata
2021-02-16 8:12 ` Michal Hocko [this message]
2021-02-16 21:53 ` David Rientjes
2021-02-17 7:54 ` Michal Hocko
2021-02-17 14:59 ` Shakeel Butt
2021-02-16 22:30 ` Mike Kravetz
2021-02-17 7:57 ` Michal Hocko
2021-02-17 10:42 ` Eiichi Tsukata
2021-02-17 12:31 ` Michal Hocko
2021-02-17 12:40 ` Michal Hocko
2021-02-18 12:22 ` Eiichi Tsukata
2021-02-18 12:39 ` Chris Down
2021-02-16 13:38 ` Chris Down
2021-02-17 9:09 ` David Hildenbrand
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